Re: [PATCH 3/5] io_uring: add support for getdents

[Christian Brauner <brauner@kernel.org>]

On Mon, Jul 31, 2023 at 11:33:05AM +1000, Dave Chinner wrote:
> On Thu, Jul 27, 2023 at 04:27:30PM +0200, Christian Brauner wrote:
> > On Thu, Jul 27, 2023 at 07:51:19PM +0800, Hao Xu wrote:
> > > I actually saw this semaphore, and there is another xfs lock in
> > > file_accessed
> > >   --> touch_atime
> > >     --> inode_update_time
> > >       --> inode->i_op->update_time == xfs_vn_update_time
> > > 
> > > Forgot to point them out in the cover-letter..., I didn't modify them
> > > since I'm not very sure about if we should do so, and I saw Stefan's
> > > patchset didn't modify them too.
> > > 
> > > My personnal thinking is we should apply trylock logic for this
> > > inode->i_rwsem. For xfs lock in touch_atime, we should do that since it
> > > doesn't make sense to rollback all the stuff while we are almost at the
> > > end of getdents because of a lock.
> > 
> > That manoeuvres around the problem. Which I'm slightly more sensitive
> > too as this review is a rather expensive one.
> > 
> > Plus, it seems fixable in at least two ways:
> > 
> > For both we need to be able to tell the filesystem that a nowait atime
> > update is requested. Simple thing seems to me to add a S_NOWAIT flag to
> > file_time_flags and passing that via i_op->update_time() which already
> > has a flag argument. That would likely also help kiocb_modified().
> 
> Wait - didn't we already fix this for mtime updates on IOCB_NOWAIT
> modification operations? Yeah, we did:
> 
> kiocb_modified(iocb)
>   file_modified_flags(iocb->ki_file, iocb->ki_flags)
>     ....
>     ret = inode_needs_update_time()
>     if (ret <= 0)
> 	return ret;
>     if (flags & IOCB_NOWAIT)
> 	return -EAGAIN;
>     <does timestamp update>
> 
> > file_accessed()
> > -> touch_atime()
> >    -> inode_update_time()
> >       -> i_op->update_time == xfs_vn_update_time()
> 
> Yeah, so this needs the same treatment as file_modified_flags() -
> touch_atime() needs a flag variant that passes IOCB_NOWAIT, and
> after atime_needs_update() returns trues we should check IOCB_NOWAIT
> and return EAGAIN if it is set. That will punt the operation that
> needs to the update to a worker thread that can block....

As I tried to explain, I would prefer if we could inform the filesystem
through i_op->update_time() itself that this is async and give the
filesystem the ability to try and acquire the locks it needs and return
EAGAIN from i_op->update_time() itself if it can't acquire them.

> 
> > Then we have two options afaict:
> > 
> > (1) best-effort atime update
> > 
> > file_accessed() already has the builtin assumption that updating atime
> > might fail for other reasons - see the comment in there. So it is
> > somewhat best-effort already.
> > 
> > (2) move atime update before calling into filesystem
> > 
> > If we want to be sure that access time is updated when a readdir request
> > is issued through io_uring then we need to have file_accessed() give a
> > return value and expose a new helper for io_uring or modify
> > vfs_getdents() to do something like:
> > 
> > vfs_getdents()
> > {
> > 	if (nowait)
> > 		down_read_trylock()
> > 
> > 	if (!IS_DEADDIR(inode)) {
> > 		ret = file_accessed(file);
> > 		if (ret == -EAGAIN)
> > 			goto out_unlock;
> > 
> > 		f_op->iterate_shared()
> > 	}
> > }
> 
> Yup, that's the sort of thing that needs to be done.
> 
> But as I said in the "llseek for io-uring" thread, we need to stop
> the game of whack-a-mole passing random nowait boolean flags to VFS
> operations before it starts in earnest.  We really need a common
> context structure (like we have a kiocb for IO operations) that
> holds per operation control state so we have consistency across all
> the operations that we need different behaviours for.

Yes, I tend to agree and thought about the same. But right now we don't
have a lot of context. So I would lean towards a flag argument at most.

But I also wouldn't consider it necessarily wrong to start with booleans
or a flag first and in a couple of months if the need for more context
arises we know what kind of struct we want or need.